Power adapters or converters can be damaged by over voltage appearing at the input resulting from, for example, lightning, high voltage disturbance, power source instability, load dump, etc . . . . For automotive cigarette lighting adapters (CLA) or other car battery powered electronic devices, a load dump is a severe transient encountered where the battery is disconnected. As seen in FIG. 1, a load dump is generated with a time duration from several milliseconds to several hundred milliseconds and a voltage spike of 25V to 90V in a 12V system. This may damage the adapters or converters. Thus, the input voltage applied to those devices should be limited to protect them from over voltage damage.
There are generally two ways to achieve the input surge protection. Turning to FIG. 2, a first method has an input surge protection circuit 10 that uses a high voltage MOSFET Q working as a source follower. Its gate is clamped by a zener diode D to a set clamp voltage so that the source voltage will follow the gate. This approach requires external components that are difficult to be integrated into a single package. Turning to FIG. 3, a second approach adds an expensive Transient Voltage Suppressive (TVS) device at the input rail to absorb any over voltage transient. Both approaches have high cost.